This invention relates generally to highlight color imaging and, more particularly, to the formation of tri-level images for this purpose.
The invention can be utilized in the art of xerography or in related printing arts. In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a photoconductive insulating surface or photoreceptor. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not struck by radiation.
This charge pattern is made visible by developing it with toner. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction.
The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
Multi-color imaging has also been accomplished utilizing basic xerographic techniques. In this instance, the foregoing process is essentially repeated for three or four cycles. Thus, the charged photoconductive surface is successively exposed to filtered light images. After each exposure the resultant electrostatic latent image is then developed with toner particles corresponding in color to the subtractive primary of the filtered light image. For example, when a red filter is employed, the electrostatic latent image is developed with toner particles which are cyan in color. The cyan toner powder image is then transferred to the copy sheet. The foregoing process is repeated for a green filtered light image which is developed with magenta toner particles and a blue filtered light image which is developed with yellow toner particles.
Each differently colored toner powdered image is sequentially transferred to the copy sheet in superimposed registration with the powder image previously transferred thereto. In this way, three or more toner powder images are transferred sequentially to the copy sheet. After the toner powder images have been transferred to the copy sheet, they are permanently fused thereto. The foregoing color imaging process is known as full color imaging.
Another color imaging process is known as highlight color imaging. In highlight color imaging two or more different color developers are customarily employed, usually black and some other color or colors, for example, red and/or blue. In one type of highlight color imaging, a tri-level image is formed on the imaging surface utilizing a three level ROS (Raster Output Scanner) to form the tri-level image on a charge retentive surface that had previously been uniformly charged. The tri-level image comprises two image areas and a background area.
The concept of tri-level xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein, the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development system is biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In tri-level xerography, the xerographic contrast on the charge retentive surface or photoreceptor is divided three, rather than two, ways as is the case in conventional xerography. The photoreceptor is charged, typically to 900 v. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged area development, i.e. CAD) stays at the full photoreceptor potential (V.sub.ddp or V.sub.cad, see FIGS. 1a and 1b). The other image is exposed to discharge the photoreceptor to its residual potential, i e. V.sub.c or V.sub.dad (typically 100 v) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD). The background areas are exposed such as to reduce the photoreceptor potential to halfway between the V.sub.cad and V.sub.dad potentials, (typically 500 v) and is referred to as V.sub.w or V.sub.white. The CAD developer is typically biased about 100 v closer to V.sub.cad than V.sub.white (about 600 v), and the DAD developer system is biased about 100 v closer to V.sub.dad than V.sub.white (about 400 v).
Because the composite image developed on the charge retentive surface consists of both positive and negative toner a pre-transfer corona charging step is necessary to bring all the toner to a common polarity so it can be transferred using corona charge of the opposite polarity.
In conventional tri-level imaging, a three level raster output scanner (ROS) is utilized. It has three operating states (i.e. "off", "full on" and "white level" or "half power"). Two of the output levels or states (i.e. "full on" and "white level") of the three level ROS must be well controlled. As will be appreciated, a highlight color imaging system using a ROS which has to be well controlled at only one state (i.e. "full on"), particularly where the "full on" state is at a substantially lower power level than required in a conventional system is highly desirable.
Various other methods of forming composite images may be relevant to the present invention:
For example, U.S. Pat. No. 4,167,324 (Wu) issued Sept. 11, 1979 discloses a technique whereby a latent image of an original document is formed on a photoreceptor by a light lens system along a first optical path while a modulated light beam input is directed along a second optical path to the surface of a stratified stylus belt. The belt is placed in proximity to the photoreceptor and acts to provide a charge pattern on the previously formed latent image in conformity with the information in the modulated laser input.
Another method of forming such images is disclosed in U.S. Pat. No. 4,385,822 (Kanbo) issued May 31, 1983. As disclosed therein, an electromagnetic recording medium is used which enables formation of a first electrostatic latent image in one layer thereof, as well as a second, magnetic latent image in a second layer thereof. The formation of the two images are synchronized and the composite latent image is subsequently developed by a specially designed developing device.
Still another method is to utilize an ion writing station as described in IBM Technical Disclosure Bulletin, Vol. 22, No. 12, May 1980, pp. 5270-5271. For this technique, a latent electrostatic image of the original document is formed by a light/lens optical arrangement. At a downstream position an ion writing station deposits a selected charge pattern on an already discharged portion of the latent image. This charge pattern conforms to the information desired to be added to the original document image.
U.S. Pat. No. 4,774,546 issued in the name of Corona et al on Sept. 27, 1988 discloses an electrophotographic reproduction device which is capable of forming images of an original document modified by information added to or replacing information of the original. A latent image of the document is formed on a photosensitive surface and a portion of the image, in a first embodiment, is maintained at the original charge level. This fully charged section is subsequently discharged in an imagewise pattern by a compact annotator device. The annotator includes an illumination source, an addressable light modulator device such as a liquid crystal panel and a lens array for forming the modulated light pattern onto the photosensitive surface.